Rapid-wetting bentonite-calcium sulfate backfill for cathodic protection



Patented Sept. 11, 1951 NITED STATS ATENT OFFICE RAPID-WETTIN G BENTONITE-CALCIUM SULFATE BACKFILL FOR CATHODIC PROTECTION Application July 9, 1947, Serial No. 759,892

This invention relates to improved anode media for use in the galvanic protection of underground metals. It also concerns packaged anodes containing such media and protection systems using them.

In galvanic systems for the cathodic protection of pipelines and other underground structures, sacrificial anodes of a metal electronegative to the structure, usually magnesium or zinc, are buried in the earth near the structure and connected to it by electrical conductors. The resulting flow of current maintains the structure cathodic with respect to the soil and greatly minimizes its corrosion. Since magnesium tends to corrode uselessly when in direct contact with some types of soils, it is customary, when using this metal as a sacrificial anode, to bury it in a prepared bed or backfill designed to control the chemical nature of the anode environment.

The chemical requirements of an ideal backfill for magnesium anodes are various. In use, the material should have good electrolytic conductivity and yet should not be so soluble as to be leached away. It should be capable of wetting easily in ground waters, and should retain moisture during dry spells so as to avoid loss of conductivity. In order to secure maximum current eificiency from the magnesium, the backfill should tend to minimize localized, i. e. useless, corrosion of the metal. on the other hand, it should promote uniformity of attack during the useful or current-producing consumption of the magnesium. In addition, it must not polarize the anode unduly or form impervious coatings on it.

These specifications are met in part by the mixtures of bentonite and gypsum now used. However, such mixtures are disadvantageous in that they absorb ground water very slowly in service, and hence do not permit development of full galvanic current for several days after installation.

It is therefore the principal object of the present invention to provide a rapid-wetting bentonite-calcium sulfate backfill for use as an anode mediLun in cathodic protection systems.

The invention will be explained with reference to the accompanying drawing, in which Fig. 1 is a schematic vertical section showing the manner of using the new bentonite-gypsum mixture as an anode backfill in the galvanic protection of a buried pipeline; and

Fig. 2 is a vertical section through a packaged anode.

Backfills according to the invention are prepared by blending together finely divided bentonite, finely-divided calcium sulfate, and water,

11 Claims. (Cl. 204197) 2 and thereafter drying the blend at a temperature above C.

In one process, bentonite and calcium sulfate, preferably each ground to a powder, are mixed together with sufiicient water to form a smooth paste, and the paste is then dried. During the drying, it changes to a mass of coarse porous granules. In another process, the ground bentonite and calcium sulfate are mixed with a lesser proportion of water to form a briquettable moist powder, which is then formed into briquets and the latter dried.

The term bentonite as used herein means a naturally-occurring colloidal clay or volcanic ash consisting largely of the minerals sodium montmorillonite or beidellite and bein characterized by swelling extensively in water. This product is sometimes called alkali bentonite in contradistinction to so-called alkaline earth bentonites which are almost non-swelling in water and are not operable in the invention.

Gypsum, i. e. the mineral CaSO4-2H2O, is the preferred form of calcium sulfate in the invention. High-purity gypsum is not required, with the ordinary dusting plaster grade sold by building supply houses being very satisfactory. However, plaster of Paris (ZCaSOrHZO), sometimes known as calcined gypsum, and anhydrous calcium sulfate may also be used.

In the compositions, a substantial proportion of both bentonite and calcium sulfate should be present. In general, a ratio of one part by weight of bentonite to 0.25 to 4 parts of gypsum is used, 7

with 3 parts of bentonite to 1 of gypsum being perhaps most satisfactory.

The proportion of water should be at least sufficient to wet the bentonite-gypsum mixture, and may be a great deal more, if a paste is to be made and dried. However, if the mixture is to be briquetted, fairly close control of the moisture content is required to get a briquettable mass, the correct amount of water for any given mixture being determined by trial. For example, with a mixture of 3 parts of bentonite and about 1 part of gypsum, there should be between about 0.36 and about 0.6 part of water.

The backfills of the invention ordinarily consist only of bentonite and calcium sulfate. However, minor proportions of other non-acidic electrolytes or inert fillers may be added if desired. For example, in backfills to be used in high-resistance soils, hydrous sodium sulfate may be incorporated in the mixture. In acidic soils, the

addition of magnesium hydroxide to maintain a1- 1 kalinity' is advantageous.

The order in which the ingredients are mixed is not critical, although best results seem to be obtained when the dry ingredients are first mixed thoroughly and water is then added.

The temperature at which the moistened bentonite-calcium sulfate mixture is dried must exceed 100 C. in order to obtain a rapid-wetting product. In general, temperatures of at least 125 C. are most satisfactory, with the range of 135 to 185 C. being preferred when briquets are made. Higher temperatures, up to 600 C., have been tried experimentally without serious damage to the product. The time of drying is merely that required to reduce the moisturecontent to a low value, being only to minutes at the preferred temperatures.

The nature of the chemical and physical change which occurs during the wetting and subsequent calcining of bentonite-calcium sulfate mixtures is not clear, but the treatment produces a very pronounced effect on the wetting rate of the product. The product made in this way appears. to be unique, since mixtures of bentonite or calcined bentonite with calcined gypsum do not exhibit similar properties. Likewise, bentonitegypsum mixtures .calcined without first being wetted are unlike the product of the invention.

The manner of using the new compositions as a cathodic protection backfill is illustrated in Fig. 1, in which a steel pipeline 4 buried in earth is being protected. The consumable galvanic anode 5 is in elongated cylindrical body of magnesium provided with a central iron core 6 which terminates in an electrical connection 1. As shown, the anode is buried in the earth near the pipeline, with the core 6 being connected electrically to the line by a conductor 8. A bed of wetted and calcined bentonite-gypsum mixture 9 surrounds the anode and is in firm contact with it and with the earth.

In making the installation, a suitable hole is dug and the anode is lowered in place, after which the backfill is tamped around it. The electrical conductor to the pipeline is then installed and buried. In dry soils, water is then poured around the anode and backfill to hasten beginning of electrolytic action.

An optional method of installation, which is particularly convenient under some conditions, involves the packaged anode illustrated in Fig. 2. In this case, a magnesium anode 5 provided with electrical connector 1 is centered in a water-permeable container, such as a paper carton Ill. The space between the anode and. the container walls is then filled with a porous mass 9 of calcined wetted bentonite-gypsum backfill. In installing this anode, it is necessary only to dig a hole the size of the carton l0, insert the entire package, tamp the earth around it, and make the necessary electrical connection.

In field use of the invention, the number and size of anodes and the quantity of backfill required to secure effective cathodic protection of a given pipeline or other structure is determined by well-known engineering principles.

While the invention has been described as useful in the cathodic protection of underground ferrous metal structures, it is applicable in protecting underground structures of any corrodible metal cathodic to magnesium. The sacrificial anodes may be made either of magnesium or of a magnesium-base alloy, both being comprehended by the term magnesium metal as used in the claims.

t The following examples are further illustra- Example 1 A mixture of 3 parts of ground bentonite and one part of ground gypsum was moistened with 10 percent of water, briquetted on a rotary briquetter, and then dried by calcining 10 to 15 minutes to a product temperature of 135 C. in a rotary drier. The briquets were then crushed to give a material passing through a A-inch opening. When immersed in water, this material wetted throughout in 5 minutes.

In comparative tests, not according to the invention, briquets prepared as above, when crushed without previously being calcined, showed in water, a wetting time of 2 hours. An unbriquetted, uncalcined mixture of 3 parts bentonite and 1 part gypsum, in water, was only about half wetted after 24 hours.

Example 2 In cathodic protection tests, identical magnesium anodes were buried in moist earth near one another and connected to the same structure to be protected. One was surrounded by a backfill according to the invention prepared by calcining at 135 C. an aqueous paste of 3 parts of bentonite and 1 part of gypsum. The other was surrounded by the same size backfill of the same bentonite-gypsum mixture which had not been wetted and calcined.

In the case of the anode with the calcined backfill, the electrical resistance between the anode and the surrounding earth within 2 minutes after installation was 12 ohms. It remained at that value for several days. With the other anode, the resistance after 2 minutes was ohms. It then declined very gradually over a period of 24 hours to 40 ohms, at which value it remained for several days. The stabilizing effect of the calcined backfill, as well as the lower overall resistance, is apparent.

What is claimed is:

1. The method of forming a rapid-wetting backfill for use in cathodic protection which comprises blending together one part by weight of finely divided bentonite, from 0.25 to 4 parts of finely-divided calcium sulfate, and water, and thereafter drying the blend at a temperature above C. but below 600 C.

2. The method of making a rapid-wetting granular backfill for use in cathodic protection which comprises blending together one part of bentonite, from 0.25 to 4 parts of gypsum, and water to form a, smooth paste, and thereafter drying the paste at a temperature above 100 C. but below 600 C.

3. A process according to claim 2 wherein the ratio of bentonite t0 gypsum is about 3:1 and wherein the mixture is dried at at least C.

4. The method of making a rapid-wetting briquetted backfill for use in cathodic protection which comprises blending together one part of bentonite, from 0.25 to 4 parts of calcium sulfate, and water in a proportion suflicient to form a briquettable mass, briquetting the latter, and drying the briquets at a temperature above 100 C. but below 600 C.

5. The method of making a rapid-wetting briquetted backfill for use in cathodic protection which consists in blending together 3 parts of bentonite, about 1 part of gypsum, and from 0.36 to 0.6 part of water, briquetting the blend, and drying the briquets at a temperature between and C.

6. A rapid-wetting backfill composition for use in cathodic protection prepared according to the process of claim 1.

7. A rapid-wetting backfill composition for use in cathodic protection prepared according to the process of claim 2.

8. A rapid-wetting briquetted backfill composition for use in cathodic protection prepared according to the process of claim 4.

9. In combination with an underground structure of a metal cathodic to magnesium, a cathodic protection system comprising a magnesium metal anode buried in the earth near the structure and electrically connected thereto, such anode being surrounded by and in intimate contact with a bed of backfill material prepared according to the process of claim 1.

10. A packaged anode for use in cathodic protection systems comprising a water-permeable container having therein a magnesium metal anode provided with means for connecting an electrical conductor thereto, such anode being surrounded by and in intimate contact with a backfill prepared according to the process of claim 1.

11. A backfill composition for use in cathodic 25 protection comprising a dried mixture of initially moistened bentonite and from to 400 per cent thereof of initially moistened calcium sulfate, the mixture having been dried at a temperature between C. and 600 C.

CLARENCE A. PIPPIN. MELVIN O. ROBINSON. WALLIS R. BENNETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Corrosion, vol. 1, No. 2 June 1945, page 68. Cements, Limes, and Plasters, by Eckel, published in 1928 by John Wiley 8: Sons, page 36. 

1. THE METHOD OF FORMING A RAPID-WETTING BACKFILL FOR USE IN CATHODIC PROTECTION WHICH COMPRISES BLENDING TOGETHER ONE PART BY WEIGHT OF FINELY DIVIDED BENTONITE, FROM 0.25 TO 4 PARTS OF FINELY-DIVIDED CALCIUM SULFATE, AND WATER, AND THEREAFTER DRYING THE BLEND AT A TEMPERATURE ABOVE 100* C. BUT BELOW 600* C. 